1. Field of the Invention
This invention pertains generally to devices and methods for controlling focus and tracking errors during readout of optical media. More particularly, the present invention is an optical focus and tracking servo system which maintains a focused servo spot on a dedicated guide or tracking surface while a focused data read/write spot is tightly coupled to the servo spot at an adjustable offset distance in both the focus and tracking directions. Servo marks are embedded in the recording medium in a format hologram to provide focus and tracking information for positional correction of the read write spot.
2. Description of the Background Art
Optical data storage provides a convenient way to achieve high storage densities in relatively inexpensive media. During recording and readout of optical data, it is important to keep the focus of the read/write beam properly positioned. Various focus and tracking error correction systems have been created in this regard to maintain correct positioning of the read/write beam focus within optical storage media. As increased data storage densities and multiple data storage layers are used in optical media, improved focus and tracking error correction has become necessary. There is presently a need for an optical data storage system with focus and tracking error correction suitable for use with multilayer data storage media, including bulk media in which data are recorded throughout the volume thereof.
The present invention comprises a focusing and tracking error correction system for multilayer optical storage media, and particularly for optical media having a format hologram recorded therein, and with data recorded in the medium as localized alterations in the format hologram at data storage locations. Data storage media of this type are disclosed in pending U.S. patent application Ser. No. 09/016,382, filed on Jan. 30, 1998, to inventor Hesselink et al., and entitled xe2x80x9cOptical Data Storage By Selective Localized Alteration of a Format Hologram and/or Retrieval by Selective Alteration of a Holographic Storage Mediumxe2x80x9d and U.S. patent application Ser. No. 09/229,457 filed on Jan. 12, 1999, in the name of inventors Daiber et. al., entitled xe2x80x9cVolumetric Track Definition for Data Storage Media Used to Record Data by Selective Alteration of a Format Hologramxe2x80x9d, the disclosures of which are incorporated herein by reference.
The present invention achieves a robust servo lock using a separate focusing and tracking system with a servo spot tracking a dedicated tracking or guide surface. The focus of the main read/write beam is tightly coupled to the servo spot, but may additionally be adjusted optically or electronically positioned at a fixed offset distance from the servo spot in both the focus and tracking directions. Focus and tracking offset distances are continually updated via error signals obtained from embedded servo bursts or marks within the recording medium.
In general terms, the invention comprises a read/write (R/W) beam focused on a data plane via a first lens, a servo or guide beam focused on a tracking plane in an optical medium via the objective lens and a second lens, a data detection system associated with the R/W beam, a servo beam detector system associated with the servo beam, a wide band tracking servo system interfaced with the objective lens and the servo beam detector, and a narrow band focusing servo system interfaced with the second lens and the data detector.
In operation, the servo beam focus spot follows the tracking layer of an optical medium, while the R/W focus spot follows embedded servo marks or bursts within a recording or data plane of the medium at a specific distance from the servo focus spot. The wide band servo tracking system defines a servo loop which, when closed, locks the R/W focus spot at a specific distance from the servo focus spot. The second lens is generally positioned so that the servo beam is divergent and focused on the tracking layer. Movement of the second lens results in movement of the servo focus spot away from the tracking layer. The second lens is servo-linked to the narrow band servo system, and undergoes corrective motion in response to focus and tracking error signals sent from the data detector to the narrow band servo system.
The focus and tracking error correction system is particularly useful for multilayer optical data storage media which include a recording layer, a wavelength specific absorbing buffer layer or layers, and a reflective or partially reflective tracking layer. In the recording layer, a format hologram is first recorded via interference of a signal beam and reference beam. An additional reflective layer may be included in the medium to facilitate recording of the format hologram. Servo marks are recorded in the recording layer of the medium by creating localized alterations or deletions in the format hologram. The servo marks are xe2x80x9cembeddedxe2x80x9d within the data or recording layer. In a preferred embodiment, the embedded servo marks are recorded prior to data recording so that the servo marks can assist in data recording. The servo marks include servo focus bursts and servo tracking bursts to maintain the R/W focus beam in the correct data track and in the correct data plane during data writing and retrieval. The embedded servo marks preferably utilize vertically staggered focus burst pairs and horizontally staggered tracking burst pairs.
The present invention also comprises optical data storage media which are structured and configured for use with the focus and servo tracking system of the invention.
Various advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing the preferred embodiment of the invention without placing limitations thereon.